U.S. patent application number 13/798895 was filed with the patent office on 2013-08-15 for composite film for board level emi shielding.
This patent application is currently assigned to HENKEL CORPORATION. The applicant listed for this patent is HENKEL CORPORATION. Invention is credited to Chih-Min Cheng, George Thomas, Bo Xia.
Application Number | 20130207005 13/798895 |
Document ID | / |
Family ID | 45994675 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130207005 |
Kind Code |
A1 |
Cheng; Chih-Min ; et
al. |
August 15, 2013 |
COMPOSITE FILM FOR BOARD LEVEL EMI SHIELDING
Abstract
An EMI shielding composite film for use in printed circuit
boards has at least two layers, a top layer electrically conductive
in all directions (isotropic), and a bottom layer electrically
conductive only in the Z (thickness) direction (anisotropic) after
thermo-compression. The bottom layer is in contact with the
grounding pads of the circuitry of the electronic device to be
shielded. The conductive top layer functions similarly to metallic
boxes to prevent the electromagnetic radiation from both entering
the boxes and escaping into the environment. The bottom layer
interconnects the top conductive layer to the grounding pads on the
PCB after thermo-compression so that electromagnetic waves
collected by the top layer are directed and released to PCB
grounding pads through the bottom layer.
Inventors: |
Cheng; Chih-Min; (Westford,
MA) ; Xia; Bo; (Irvine, CA) ; Thomas;
George; (Bergenfield, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HENKEL CORPORATION; |
|
|
US |
|
|
Assignee: |
HENKEL CORPORATION
Rocky Hill
CT
|
Family ID: |
45994675 |
Appl. No.: |
13/798895 |
Filed: |
March 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US2011/057418 |
Oct 24, 2011 |
|
|
|
13798895 |
|
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61406705 |
Oct 26, 2010 |
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Current U.S.
Class: |
250/515.1 |
Current CPC
Class: |
H05K 9/0088 20130101;
G21F 1/00 20130101 |
Class at
Publication: |
250/515.1 |
International
Class: |
G21F 1/00 20060101
G21F001/00 |
Claims
1. A composite film for EMI shielding having at least two layers, a
top layer electrically conductive in all directions (isotropic),
and a bottom layer electrically conductive only in the Z direction
(anisotropic) after thermo-compression.
2. The composite film according to claim 1 in which the top layer
comprises a polymeric resin filled with conductive filler particles
at a loading level effective to establish isotropic
conductivity.
3. The composite film according to claim 2 in which the top layer
polymeric resin comprises at least one thermoset resin, or at least
one thermoplastic resin, or a combination of thermoset and
thermoplastic resins.
4. The composite film according to claim 3 in which the top layer
thermoset resins are selected from the group consisting of vinyl,
acrylic, phenolic, epoxy, maleimide, polyimide, or
silicon-containing resins, and the top layer thermoplastic resins
are selected from the group consisting of acrylics, phenoxy resins,
thermoplastic polyesters, polyamides, polyurethanes, polyolefins,
polysulfide rubber, and nitrile rubbers.
5. The composite film according to claim 2 in which the top layer
conductive filler particles are selected from the group consisting
of silver, nickel, copper, graphite, carbon nanotubes, and
core/shell particles, in which core/shell particles the core is
selected from the group consisting of silica, glass, boron nitride,
metal, polyethylene, polystyrene, phenol resin, epoxy resin, acryl
resin and benzoguanamine resin and the shell is selected from the
group consisting of silver, nickel, and copper.
6. The composite film according to claim 1 in which the top layer
conductive filler loading is 15 volume percent or greater with
respect to the total composition of the top layer.
7. The composite film according to claim 1 in which the top layer
is a metal foil or a metal mesh, or a combination of a metal foil
or a metal mesh and a polymeric resin filled with conductive
particles.
8. The composite film according to claim 1 in which the bottom
layer comprises an adhesive polymeric resin filled with conductive
particles at a loading level effective to establish anisotropic
conductivity upon the application of thermo-compression.
9. The composite film according to claim 8 in which the bottom
layer polymeric resin comprises at least one thermoset resin, or at
least one thermoplastic resin, or a combination of thermoset and
thermoplastic resins, in which the bottom layer is substantially
dry to the touch.
10. The composite film according to claim 9 in which the bottom
layer thermoset resins are selected from the group consisting of
vinyl, acrylic, phenolic, epoxy, maleimide, polyimide, and
silicon-containing resins and the bottom layer thermoplastic resins
are selected from the group consisting of acrylics, phenoxy resins,
thermoplastic polyesters, polyamides, polyurethanes, polyolefins,
polysulfide rubber, and nitrile rubbers.
11. The composite film according to claim 8 in which the bottom
layer conductive filler loading level is 2 to 20 volume percent
with respect to the total composition of the bottom layer.
12. The composite film according to claim 8 in which the bottom
layer conductive filler particle diameter is within the range of 1
.mu.m to 125 .mu.m.
13. The composite film according to claim 8 in which the bottom
layer conductive filler is selected from the group consisting of
silver, copper, nickel, graphite, and core shell particles, in
which the core/shell particles have a conductive shell and a
conductive or dielectric core.
14. The composite film according to claim 13 in which the bottom
layer conductive filler is core/shell particles selected from the
group consisting of gold coated polymer spheres, silver coated
silicate, tungsten carbide (WC) coated aluminum, and graphite
coated metal.
15. The composite film according to claim 8 in which the bottom
layer further comprises dielectric fillers with a particle size
smaller than the particle size of the conductive filler, selected
from the group consisting of boron nitride, aluminum oxide,
aluminum nitride, and particles coated with these materials, and
present at a loading level in the range of 10 weight percent to 80
weight percent based on the composition of the bottom layer.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a film for shielding electronic
devices, such as, computers, communication devices, printers, video
cameras, and the like, from emitting electromagnetic radiation
(EMI).
[0002] Electronic devices emit electromagnetic radiation that can
interfere with television, radio, and other communications. The
level of EMI is regulated by governments, and consequently the
manufacturers of electronic devices are required to limit the level
of EMI produced by their devices. A second reason for limiting EMI
is that stray signals within a device can cause internal
interference or cross-talk. Two approaches are used currently to
limit EMI: suppressing the electromagnetic radiation at the source,
or containing the radiation so that it does not escape the
device.
[0003] Containment can be accomplished per Faraday's principle by
enclosing the emitting device in a perfectly conducting shield,
such as, a metallic can or a conformal coating. However, the
metallic can is less than optimum because there are always areas
from which radiation can escape, it adds cost and weight to the
electronic device, and it is not suitable for a flexible substrate.
Moreover, if rework is needed, the metallic can must be de-soldered
and then re-soldered, which increases the risk of damaging active
devices.
[0004] Conformal coatings also have disadvantages. They are
typically applied in multiple layers, a dielectric insulating layer
and a conductive layer, which require multiple processing steps.
The conductive layer is usually applied as a liquid ink and if not
carefully controlled may lead to deposition in an undesired area
and cause shorts in the circuitry. The drying/curing time for
printed conductive inks is in the range of 10 to 30 minutes, longer
than desired, and conductive inks may contain volatile organic
solvent. The dielectric layer is interposed between the conductive
layer and the circuitry to prevent the conductive layer from
electrically contacting predetermined areas of the circuitry and
substrate.
[0005] In order to overcome the current disadvantages of EMI
shielding, the instant invention is disclosed and claimed.
SUMMARY OF THE INVENTION
[0006] This invention is a composite film, which shields for EMI,
for use in fabricating printed circuit boards (PCB). The film has
at least two layers, a top layer electrically conductive in all
directions (isotropic), and a bottom layer electrically conductive
only in the Z (thickness) direction (anisotropic) after
thermo-compression. (Thermo-compression is the application of heat
and pressure.) The bottom layer is in contact with the grounding
pads of the circuitry of the electronic device. The conductive top
layer functions similarly to metallic boxes to prevent the
electromagnetic radiation from both entering the boxes and escaping
into the environment. The bottom layer interconnects the top
conductive layer to the grounding pads on the PCB after
thermo-compression so that electromagnetic waves collected by the
top layer are directed and released to PCB grounding pads through
the bottom layer.
[0007] The level of conductive filler in the bottom layer is below
the level that would cause electrical shorting in the circuitry of
the device without the application of thermo-compression. That is,
for those areas of the substrate and circuitry outside of the
grounding pads and consequently not subject to thermo-compression
processing, the conductive filler in the bottom layer is at too low
a level to allow conductance. However, when thermo-compression
processing is applied to localized areas, the pressure and heat at
those localized areas cause the conductive fillers in those
localities to sinter and interconnect, thereby allowing the
connection of the active device to the top conductive layer. The
level of thermo-compression is an effective level to cause the
interconnection of the conductive filler particles between the top
layer and the bottom layer.
[0008] The conductive top layer functions similarly to metallic
cans or metal cases, and contains an effective amount of conductive
filler to prevent the ingress or egress of electromagnetic
radiation (without thermo-compression).
DETAILED DESCRIPTION OF THE INVENTION
[0009] The top layer of the EMI shielding film can be composed in
alternative embodiments establishing isotropic conductivity. In one
embodiment the top layer comprises a polymeric resin filled with
conductive particles at a loading level effective to establish
isotropic conductivity. The polymeric resin comprises at least one
thermoset resin, and/or at least one thermoplastic resin. Exemplary
suitable thermoset resins include vinyl, acrylic, phenolic, epoxy,
maleimide, polyimide, or silicon-containing resins. Exemplary
suitable thermoplastic resins include acrylics, phenoxy resins,
thermoplastic polyesters, polyamides, polyurethanes, polyolefins,
polysulfide rubbers, and nitrile rubbers.
[0010] The conductive filler particles for the top layer can be any
effective filler at any effective loading to provide isotropic
conductivity. Suitable fillers include silver, nickel, copper,
graphite, carbon nanotubes, or core/shell particles. If core/shell
particles are used, the core can be an inorganic particle, such as
silica, glass, boron nitride, or metal, or it can be an organic
resin, such as polyethylene, polystyrene, phenol resin, epoxy
resin, acryl resin or benzoguanamine resin; the shell can be a
conductive element such as silver, nickel, or copper.
[0011] Suitable conductive filler loading levels are 15 volume
percent or greater, depending on the shape and size of the
conductive fillers, with respect to the total composition of the
top layer. Silver coated copper (Ag/Cu) is suitable.
[0012] In another embodiment, the top layer can be a metal foil or
a metal mesh, such as, for example, copper or aluminum. In a
further embodiment, the top layer can be a combination of a metal
foil or metal mesh and the polymeric resin filled with conductive
particles.
[0013] The bottom layer of the EMI shielding film will be
sufficiently adhesive to connect the composite film to the EMI
shielded components or substrate. The bottom layer comprises an
adhesive polymeric resin filled with conductive particles at a
loading level effective to establish anisotropic conductivity upon
the application of thermo-compression. The bottom layer polymeric
resin comprises at least one thermoset resin, and/or at least one
thermoplastic resin. Exemplary suitable thermoset resins include
vinyl, acrylic, phenolic, epoxy, maleimide, polyimide, or
silicon-containing resins. Exemplary suitable thermoplastic resins
include acrylics, phenoxy resins, thermoplastic polyesters,
polyamides, polyurethanes, polyolefins, polysulfide rubber, and
nitrile rubbers.
[0014] The conductive fillers for the bottom layer are loaded
typically at 2 to 20 volume percent (vol %) with respect to the
total composition of the bottom layer. In one embodiment, the
conductive filler for the bottom layer is present in an amount of
about 1 to about 5 volume percent.
[0015] At loadings within this range, the particles are
sufficiently dispersed in the polymeric resin so as not to contact
each other laterally, thus avoiding x-y conductivity. The bottom
layer filler particle diameter is selected to be smaller than the
bottom layer thickness. Suitable particle diameters are in the
range of 1 .mu.m to 125 .mu.m. Suitable bottom layer fillers
include silver, copper, nickel, and graphite. Conductive filler
with conductive shell and conductive or dielectric core can also be
used. Examples include gold coated polymer spheres, Ag coated
silicate, tungsten carbide (WC) coated aluminum, and graphite
coated metal. Other suitable bottom layer fillers include silver,
copper, nickel, and graphite with a dielectric outer coating to
further ensure no possibility of circuitry shorting. If such a
dielectric outer coating is used, it should be selected to breakup
easily with pressure or melt away with heat, so that when
thermo-compression is applied in localized areas, conductive
interconnections can be formed.
[0016] In addition to the electrically conductive fillers, the
bottom layer may also contain thermally conductive but electrically
non-conductive (dielectric) fillers to enhance thermal conductivity
of the package. Exemplary thermally conductive dielectric fillers
include boron nitride, aluminum oxide, aluminum nitride, and
particles coated with these materials. When thermally conductive
dielectric fillers are present, they are present in the range of 10
weight percent to 80 weight percent (wt %) with respect to the
total composition of the bottom layer. The thermally conductive
dielectric fillers (or other functional non-conductive fillers)
will have a maximum particle size smaller than the particle size of
the conductive fillers in the bottom layer.
[0017] In one method of preparation of the bottom layer, hot melt
resins are homogenized with conductive fillers (for example, using
a heated compounding machine). This hot melt mixture is extruded
through a slot die to a given thickness and the extruded film
calendared further for reduced thickness.
[0018] In another method of preparation of the bottom layer, the
bottom layer is prepared from one or more solvent-free liquid
B-stageable thermoset resins or a combination of thermoset and
thermoplastic resins. Conductive fillers are dispersed into the
liquid resins using conventional mixing vessels and blades. The
mixture is disposed directly onto the top layer, or disposed onto a
release liner. Using heat or UV radiation, the bottom layer is
cured to form a B-staged coating or film. If the bottom layer
mixture is disposed onto a release liner, after B-staging it is
contacted to the top layer, the bottom and top layers are
laminated, and the release liner is removed. This bottom layer will
be further crosslinked to provide a reliable interconnect during
the thermo-compression process occurring in the EMI assembly, or
later if there is a post-cure step.
[0019] Alternatively, the bottom layer can be prepared from a
solvent based thermoplastic or thermoset resin system. Conductive
fillers are dispersed in the solvent and resin mixture using
conventional mixing vessels and blades. The mixture is disposed
onto the top layer or disposed onto a release liner, followed by
solvent evaporation to form a film. If the mixture is disposed onto
a release liner, after solvent evaporation and film formation, the
bottom layer is contacted to the top layer, the layers are
laminated, and the release liner is later removed.
[0020] In another embodiment, the bottom layer can be prepared as a
composite of different layers in which, for example, the first
layer is a film prepared from a reactive resin, and the second
layer is a film prepared from a curing agent; optionally, a third
layer prepared from an inert material can be inserted between the
first and second layer to prevent pre-reaction between those
layers, thereby enhancing the shelf life of the composite bottom
layer film.
[0021] In all embodiments for the bottom layer disclosed herein,
the bottom layer will be substantially dry to the touch after
solvent evaporation, after thermal or UV B-staging, or after hot
melt extrusion and cooling.
[0022] The composite film of top layer and bottom layer is prepared
by laminating the conductive top layer to the bottom layer, or by
coating the bottom layer directly onto the top layer. In one
embodiment, the bottom layer thickness will be 50 .mu.m or greater,
and the top layer thickness will be between 10-100 .mu.m, depending
on requirements for film conductivity and shielding effectiveness.
In some embodiments, a third pressure-sensitive-layer can be added
below the bottom layer to control positioning during assembly
processes. The composite film can be slit to the desired width and
cut to the desired length, and can be packaged on a reel.
[0023] Utilization of the composite film as an EMI shield for
printed circuit boards (PCB) can occur in several embodiments. In
one embodiment, after all the PCB components are soldered and
functional testing has been completed, the EMI shield composite
film is picked-up and placed on top of the PCB. The film is
softened by heating the assembly from the top and/or bottom to a
temperature 30.degree. to 50.degree. C. above the softening
temperature of the composite film. The softened film will conform
to the contour of the components needing EMI shielding protection.
A hot air stream or a heated metal block with contours matching the
PCB layout can be used as the heat source. Interconnects between
the conductive top layer to the PCB ground pads is established by
thermo-compression, that is, by thermally compressing the composite
film to the PCB ground pad with a heated source, such as, a heated
metal bar or a high pressure hot air stream. In one embodiment, the
film softening/conforming and interconnect are done in a single
step.
[0024] In a further embodiment, if functional testing of the PCB is
not required before application of the EMI shielding composite
film, the composite film can be picked-up and placed to the desired
substrate and the softening/conforming step can be done together
with solder reflow and electrical connection of all the components.
During the cooling period after the solder reflow process, a metal
bar can be used to establish the interconnect as described above.
This assembly scenario is fully compatible with the current
metallic can process.
[0025] In another embodiment, if the choice of filler is one that
is sharp and hard, for example, tungsten carbide coated aluminum,
and the bottom dielectric film has proper softness at room
temperature, then a cold stamping process, followed by heat curing
the dielectric resin, can be used to make the interconnect between
the top conductive layer and PCB ground pads. The heat cure
following the cold stamping secures the interconnect and provides
strong adhesion. In this case the thermo-compression step is not
needed.
[0026] In one embodiment, a catalyst or accelerator can be
dispensed to the PCB ground pad before placement of the EMI
shielding composite film to further improve the cure speed of the
bottom layer.
[0027] The proposed composite film may be cramped into a wavy
format. The advantage of a wavy format film compared to a flat
format film is to provide expansion, which better accommodates any
three dimensional electronic components underneath the EMI
composite film.
* * * * *